Plasma etch and deposition processes have become the dominant pattern transfer means used in semiconductor manufacturing over the past 20 years. Most plasma based processes employ the fundamental principle of disassociation of a feed gas by the application of radio frequency (RF) power.
As with all plasma loads, one of the dominant characteristics of the plasma load is its non-linearity. The non-linearity of the load affects the voltage and current sine waves of the delivered RF power by creating prevalent harmonic distortion. The exact amount of harmonic distortion, as represented by the amplitude of the harmonic frequencies and the associated phase angle of the current harmonic relative to the corresponding voltage harmonic, is unique to the plasma creating them. To be more precise, the plasma parameters, including ion and electron densities and energies, collision frequencies, neutral constituents, and their respective densities all contribute in a unique way to the amplitude of specific harmonic components of the fundamental frequency applied by a power delivery source to achieve the desired disassociation and subsequent process results.
It is thus apparent that, by monitoring the harmonic components of the fundamental frequency in the delivered RF power, enhanced process control of plasma deposition and etch processes may be obtained. Consequently, various sensors have been developed in the art for this purpose.
Conventional RF sensors require that the current carrier be physically disrupted and reconnected at the terminals of the sensor. This has the effect of adding physical and electrical length to the current carrier due to the addition of the internal conductor of the sensor to the current path. Moreover, the addition of conventional RF sensors to a system frequently requires the addition of even further current carrier to allow the system components to be rearranged so as to avoid interference between the RF sensors and other system components, and to allow for proper grounding of all of the components.
While the addition of current carrier may be tolerable in some lower frequency applications, many plasma reactors today are designed to operate at fundamental frequencies that are in the megahertz region (and at harmonics of these frequencies that are often up to 300 megahertz). At these frequencies, the addition of even a few centimeters of current carrier, and the associated increase in electrical length, is significant in terms of the added capacitance and the shift in phase angle that it induces. Indeed, it is often necessary, after these RF sensors have been installed, to add additional capacitance to the RF current carrier so that the phase angle will be rotated around the Smith chart, thereby compensating for this effect. The need for such adjustments unduly complicates the installation of the sensor and can require recalibration of system parameters, since the calibration coefficients determined for the sensor at the point of manufacture may no longer be accurate.
There is thus a need in the art for an RF sensor, suitable for use with plasma reactors, which does not add to the electrical length of the RF current carrier. There is further a need in the art for a method and device for incorporating an RF sensor into the RF current carrier that does not require the current carrier to be broken, and that does not necessitate the addition of electrical length to the current carrier. There is also a need in the art for a method for measuring RF frequencies that does not significantly modify the attributes of the RF current. These and other needs are met by the devices and methodologies disclosed herein.